Number 2 • 2015

#850

Limiting Medusahead Invasion and Impacts in the Great Basin

Medusahead (Taeniatherum caput-medusae) is an exotic winter annual grass from Eurasia, and was first reported in Purpose: To provide managers with strategies to North America in the 1880s. It occurs across a broad range of reduce the spread and impact of medusahead. climatic and soil conditions. Medusahead can occur on sites receiving from 250 to 1000 mm (10-40 in) of precipitation. Medusahead is most problematic on fine-textured soils below In Brief: 1524 m (5000 ft), but can occur at higher elevations and on more coarse-textured, well-drained soils. • Medusahead invasions decrease biodiversity, degrade wildlife habitat, reduce livestock forage, It is critical to limit the spread and impact of medusahead increase the risk of frequent wildfires, and change invasion because it decreases biodiversity, degrades wild- how ecosystems function. life habitat, reduces livestock forage, increases the risk of frequent wildfires, and changes how ecosystems function • Seed dispersal occurs primarily via vehicles and (Young 1992; Davies and Svejcar 2008; Davies 2011). There animals. are three primary tactics to limiting medusahead invasion • Short-distance dispersal can be reduced by and subsequent negative impacts: 1) reduce seed dispersal, 2) applying selective herbicides, and planting maintain or increase plant community resistance to invasion, competitive vegetation (such as perennial and 3) use early detection and eradication of new infestations grasses) around infestations. in non-invaded areas. • Long-distance dispersal requires limiting contact Reducing Seed Dispersal with vectors, maintaining “weed-free” zones, and Most medusahead seeds only disperse a few meters (Davies controlling livestock rotations in infested areas. 2008) from the parent plant. Longer distance seed dispersal happens primarily by humans (often via vehicles) and ani- mals (Davies et al. 2013). Strategies for reducing short-distance dispersal: • Reducing short-distance spread can be accomplished by applying selective herbicides around infestations. Applying pre-emergent herbicides in the fall can effectively control medusahead and minimize damage to perennial vegetation. • Planting competitive vegetation, such as crested wheat- grass (Agropyron cristatum), around the infestations can also reduce the spread (Davies et al. 2010). Strategies for reducing long-distance dispersal: Reducing long-distance dispersal requires limiting contact by vehicles, animals, and humans with medusahead seeds and cleaning seeds off when contact occurs. • Maintaining medusahead-free zones (usually with herbi- cides) along roads and trails can reduce the spread of medu- sahead seeds. Figure 1: Medusahead seed head in an invaded area. - 1 - • If medusahead-free zones cannot be maintained, some roads may need to be closed during times when seeds can be readi- ly dispersed. • Vehicles (especially fire suppression, off-road, and con- struction equipment) and gear, clothes, and shoes, should be cleaned after travelling through or working in medusahead invaded areas. • Livestock should not be moved directly from infested fields to un-invaded areas. Pasture rotations should minimize livestock contact with infestations when seeds can be readily dispersed. Resisting Medusahead Invasion The composition of the plant community is critical in de- termining resistance to medusahead invasion. In the Great Basin, a reduction in medusahead establishment is linked to increases in perennial bunchgrass abundance (Figure 2). It is necessary to maintain intact perennial bunchgrass commu- nities and restore degraded bunchgrass communities to limit medusahead invasion. • Carefully managed livestock grazing is crucial to maintain resistance to medusahead invasion. Livestock grazing during the growing season should be moderate (~40% utilization) Figure 2. Relationship between medusahead densi- or less. Managers should avoid repeated use over growing ty and perennial grass density. Adapted from Davies seasons and incorporate periods of grazing rest. Complete (2008). TACA = medusahead, PG = perennial grasses, grazing exclusion likely has varying effects, but the accumu- AG = annual grasses excluding TACA. lation of fine fuels in the absence of livestock grazing may increase fire risk, potential fire severity, and post-fire annual • Priority for surveying should concentrate first along roads, grass invasion in some situations (Davies et al. 2009). secondarily along animal trails, and then at random locations, • Managers need to minimize disturbances (e.g., construction, because roads are most likely the vector pathway for medu- catastrophic wildfire, non-selective herbicide application) sahead spread into new areas (Davies et al. 2013). Roadsides that reduce the perennial herbaceous understory, because this often are also an ideal environment for medusahead estab- will increase the probability of medusahead invasion. lishment. • Perennial bunchgrasses need to be re-established after • Managers might prioritize a survey near an existing infesta- disturbances that result in significant bunchgrass mortality, tion, because medusahead seed abundance and likelihood of otherwise medusahead or other exotic annual species may fill establishing new infestations are much greater near existing open spaces in the plant community. infestations. • Managers can monitor trends in bunchgrass abundance, and • They should survey more extensively within recently dis- improve management if a negative trend is detected. turbed areas where susceptibility to invasion is greater. Early Detection and Eradication • New infestations in uninvaded areas should be targeted for eradication. Management to limit the dispersal of medusahead, and increase the resistance of plant communities to invasion is • Areas that received control treatments should be routine- highly effective, but will not prevent all medusahead es- ly monitored to ensure treatments are successful and are tablishment opportunities in previously uninvaded areas.It re-treated as necessary. Revegetation may be necessary to is very important to detect new infestations and implement prevent re-invasion of treated areas. management plans to eradicate them. Limiting the spread and impact of medusahead requires • A survey plan that outlines inventory techniques, the survey involvement from all stakeholders. It is vital that stakehold- area, and survey time periods is critical for success (Sheley et ers are informed about medusahead impacts and the efforts al. 2003). needed to prevent spread and negative impact.

- 2 - Authors Davies, K.W., A.M. Nafus, and R.L. Sheley. 2010. Non-na- tive competitive perennial grass impedes the spread of an Kirk W. Davies invasive annual grass. Biological Invasions 12:3187-3194. Agricultural Research Service, Burns Unit [email protected] Davies, K.W., and T.J. Svejcar. 2008. Comparison of me- dusahead-invaded and noninvaded Wyoming big sagebrush Dustin Johnson steppe in southeastern Oregon. Rangeland Ecology & Man- Department of Animal and Rangeland Sciences, OSU agement 61:623-629. [email protected] Davies, K.W., T.J. Svejcar, and J.D. Bates. 2009. Interaction References of historical and non-historical disturbances maintains native Davies, K.W. 2008. Medusahead dispersal and establishment plant communities. Ecological Applications 19:1536-1545. in sagebrush steppe plant communities. Rangeland Ecology Sheley, R.L., S. Kedzie-Webb, and B.D. Maxwell. 2003. & Management 61:110-115. Integrated weed management on rangelands. Pages 57–68 in Davies, K.W. 2011. Plant community diversity and native Sheley, R.L. and J.K. Petroff, eds. Biology and management plant abundance decline with increasing abundance of an of noxious rangeland weeds. Oregon State University Press. exotic annual grass. Oecologia 167:481-491. Corvallis, OR. Davies, K.W., A.M. Nafus, and M.D. Madsen. 2013. Medu- Young, J.A. 1992. Ecology and management of medusahead sahead invasion along unimproved roads, animal trails, and (Taeniatherum caput-medusae ssp. asperum (Simk.) Melde- random transects. Western North American Naturalist 73:54- ris). Great Basin Naturalist 52: 245-252. 59.

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